Relief valves are used to control or limit the pressure in a system or vessel which can build up in the vessel. In particular, relief valves are used on vessels such as truck containers carrying cryogenic liquids such as liquefied natural gas to allow the pressurized gases to flow from the relief valve in the event of a pressure buildup to prevent failure of the container. Cryogenic liquids stored in containers pose a particular challenge because pressure is always building in the container as the cryogenic temperature is around −295° F. Present pressure relief systems used with cryogenic liquid containers typically contain three relief valves: a first pressure relief valve which cycles open and closed to keep the pressure controlled, a second pressure relief valve which opens to protect the vessel if the pressure continues to build even with the first valve operating, and a third burst disc valve which discharges if the first two valves cannot handle the pressure build in the container, to protect the container from catastrophic failure.
Current first and second pressure relief valves for such systems are of the direct spring actuated type, such that once the pressure on the inlet side of the valve reaches a predetermined point (the set pressure of the valve), the seal of the seat is released and the valve begins to open to release gas through an outlet and into a release vessel. The top of the valve is sealed so that the only escape for the gases relieved is safely through the outlet. If the pressure at the inlet continues to climb, the valve will pop to a full open position and gas will flow through the outlet at the valve's rated capacity. The speed at which this popping action occurs is very fast. Current valves have an outlet at 90° from the seat and spring. This poses an issue with the performance in that once the valve is at the point where it pops, a piston effect occurs. The piston effect is caused by a compression of the air in the chamber that houses the spring and internal parts that are above the seat and poppet. When the valve pops, the poppet is forced upward rapidly, compressing the column of air in the chamber which counteracts the force pushing upwards on the poppet, urging the valve to close. This process repeats since the pressure on the inlet is constantly pushing the poppet back up. What results from this repeated up and down movement of the poppet is known as chatter. Instead of the valve opening to its full open position and remaining there for the duration of the over pressure event, it constantly opens and closes very rapidly. This means the valve will not relieve at the proper rate, and it can also be damaging to the internal parts of the valve.